The production methods for shell cores and molds which were commonly used heretofore have had various economic, and physical disadvantages. However, the use of "shell" concept is advantageous because of the reduced amount of sand-binder needed.
The Kroning process, which is one of the earliest known dry "hot box" methods, is feasible for use in the production of relatively small shell cores and molds. It requires heated patterns which must be able to compensate for thermal warpage and expansion, and for the abrasion caused by the placement of sand therein. For this reason, they are usually made for iron or steel. This process involves the use of sand having coated thereon a dry solid resin, usually a phenol-formaldehyde novolac resin, catalyzed with hexamethylene tetramine. This catalyzed resin binder is stable under ordinary atmospheric ambient conditions but is catalytically polymerized, that is, cured at elevated temperatures. In the practice of the Kroning method, the dry coated catalyzed sand, usually AFS 80-90 grain fineness, for example, is packed into a metal pattern capable of being heated. The pattern is pre-heated to a sufficiently high temperature, e.g. 450.degree.-500.degree. F. to achieve the polymerization of the binder on the sand disposed thereagainst. Typically only a thin layer of the sand mixture is cured and the pattern is then inverted and the uncured sand-binder is dumped and separated from the relatively thin "shell" of cured sand-binder, and the dumped uncured sand-binder portion is salvaged for reuse.
In the so-called "cold box" method for the manufacture of shell cores and molds liquid or "wet" binders such as, for example, liquid furfuryl alcohol resin binders are used. These cold box methods require manual placement of a thin layer of the sand-binder mixture against the pattern. The use of vibrators to increase interparticulate contact requires care insofar as substantially vertical portions of the packed sand may tend to dislodge or separate. Usually, these patterns are provided with venting means for passing the catalyst gas through the entire sand mass thus curing all of the sand-binder mix therein. This method also suffers the disadvantage of requiring hand packing or hand placement of the initially used sand-binder mixture resulting in inconsistent and non-uniform sand depth dimensions, and therefore, in non-uniform shells.
Illustrative of patents which have related to these areas include U.S. Pat. No. 3,008,205 to H. O. Blaies, Jr., "Shell Type Molds and Cores", U.S. Pat. No. 2,874,428 to J. V. L. Bonney, Jr., "Method of Hardening of Shell Cores and the Like", U.S. Pat. No. 3,145,438 to R. H. Kottke, et al., "Gas Cure of Organic Bonds for Sand and Abrasive Granules", U.S. Pat. No. 3,428,110 to J. Walker, et al., "Process for Production of Foundry Cores and Molds", and U.S. Pat. No. 3,639,654 to J. Robins, for "Gaseous Halo-Sulfonic Acid Anhydride Catalysts for Curing Furfuryl Alcohol and Furan Resin".
It is an object of the present invention to provide a "cold box" method for making shell cores and molds, which method does not require the manual placement of a relatively thin layer of sand-binder mixture for complete curing thereof, and which method also provides automatically for the curing of the sand-binder mixture which is exposed to the catalytic gasses in the void space within the gassing canopy wall.